1. Field of the Invention
This invention relates to photoresist compositions, a hardened form thereof, and metal patterns therefrom that are suited for use in the fabrication of MEMS (micro-electromechanical system) components, micromachine components, microfluidic components, μ-TAS (micro total analysis system) components, ink-jet printer components, microreactor components, electroconductive layers, metallic bump connections, LIGA components, forms and stamps for microinjection molding and microembossing, screens or stencils for fine printing applications, MEMS and semiconductor packaging components, and printed wiring boards that can be processed by ultraviolet (UV) lithography.
2. Brief Description of Prior Art
Photoimageable coatings are currently used in a wide variety of semiconductor and micromachining applications. In such applications, photoimaging is accomplished by exposing the coating on a substrate to patterned radiation thereby inducing a solubility change in the coating such that the exposed or unexposed regions can be selectively removed by treatment with a suitable developer composition. The photoimageable coating (photoresist) may be either of the positive or negative type, where exposure to radiation either respectively increases or decreases the solubility in the developer. Advanced packaging applications requiring solder bumps with a high aspect ratio (defined as the height to width ratio of the imaged feature), or applications involving the fabrication of micro-electromechanical devices (MEMS) often require photoimageable compositions capable of producing uniform spin-coated films and high aspect ratio images with vertical sidewall profiles in films with a thickness greater than ten microns.
Devices in which micromechanical elements, electronic circuits including radiofrequency elements, and optical elements are assembled are called MEMS (micro-electromechanical systems) or micromachines. Many applications of these have been studied and they have been put into practical use. (see Year 2002, issued by Micromachine (Ltd.) Industrial Technology Service Center.) Although these devices, which are based on semiconductor technology, are small, they are complex and this name is generally given to devices having high-order function and represents the basic technology that constitutes important components that are keys to systems such as various types of sensors, printer heads, disk heads, communications light switches, and biochips.
Conventional positive photoresists based on diazonaphthaquinone-novolac chemistry are not well-suited to applications requiring film thicknesses greater than about 50 microns. This thickness limitation is caused by the relatively high optical absorbance of the diazonaphthaquinone-type (DNQ) photoactive compounds at wavelengths in the near-ultraviolet region of the optical spectrum (350–450 nm) which are typically used to expose the photoresist. Also, DNQ-type photoresists possess limited contrast, or differential solubility, of the exposed vs. unexposed photoresist in a developer solution which results in relief image sidewalls that are sloped rather than vertical. Optical absorption necessarily reduces the radiation intensity as it traverses from the top to the bottom of the film, such that if the optical absorption is too high, the bottom of the film will be underexposed relative to the top, causing a sloped or otherwise distorted profile of the developed image. Nevertheless, DNQ resist formulations are available for use at film thicknesses up to 100 microns, but at a great increase in the required exposure dose.
A negative, spin-coated, thick-film photoimageable composition of the chemically amplified type, which has a very low optical absorbance at wavelengths in the 350–450 nm range has been described in the literature [N. LaBianca and J. D. Gelorme, “High Aspect Ratio Resist for Thick Film Applications”, Proc. SPIE, vol. 2438, p. 846 (1995)]. High aspect ratio (>10:1) photoimaging was demonstrated in 200 micron thick films. This resist comprises a solution in a casting solvent of a highly branched, octafunctional epoxy-novolac resin, EPON® SU-8 from Resolution Performance Products, and a photoacid generator (PAG) such as CYRACURE® UVI 6974 from Dow Chemical which consists of a mixture of arylsulfonium hexafluoroantimonate salts. The resulting photoresist formulation may be spin coated or curtain coated onto a wide variety of substrates, pre-baked to evaporate solvent, leaving a solid photoresist coating of one hundred microns or greater thickness which may be photoimaged by exposure to near-ultraviolet radiation through a patterned photomask using contact, proximity, or projection exposure methods. Subsequent immersion of the imaged layer in a developer solvent dissolves away the unexposed regions, leaving behind a high resolution, negative-tone relief image of the photomask in the film.
EPON® SU-8 resin is a low molecular weight epoxy-functional oligomer that has several characteristics making it advantageous for high aspect ratio photoimaging in thick films: (1) it has a high average epoxide functionality (eight), (2) a high degree of branching, (3) high transparency at wavelengths of 350–450 nm, and (4) the molecular weight is sufficiently low as to allow preparation of high solids coating compositions. The high functionality and branching result in efficient crosslinking under the influence of strong acid catalysts, while the high transparency allows uniform irradiation through thick films, making the resist capable of forming images with aspect ratio of greater than 10:1 at film thicknesses of greater than 100 microns. In fact, only high epoxy functionality and a high degree of branching will provide the high aspect ratio structures with straight sidewalls.
Suitable photoacid generators based on sulfonium or iodonium salts are well-known and have been extensively discussed in the literature [see for example, Crivello et al., “Photoinitiated Cationic Polymerization with Triarylsulfonium Salts”, Journal of Polymer Science: Polymer Chemistry Edition, vol. 17, pp. 977–999 (1979).] Other useful PAGs with appropriate absorbance include the carbonyl-p-phenylene thioethers as described in U.S. Pat. Nos. 5,502,083 and 6,368,769. Additionally, sensitizers such as 2-alkyl-9,10-dimethoxyanthracenes or various naphthalene, peryl or pyryl compounds can be added to the formulation or incorporated into the PAG as described in U.S. Pat. No. 5,102,772. Negative photoresists based on the above disclosed compositions which are suitable for spin-coating are sold by MicroChem Corp., Newton, Mass., USA and are used commercially, especially in the fabrication of MEMS devices. For example, a product typically offered by MicroChem, “SU-8 50” can be spin-coated at 1000–3000 rpm to produce films of thickness in the range of 30–100 microns, which after exposure and solvent development, can produce images having an aspect ratio greater than 10:1 at film thicknesses greater than 100 microns. Higher or lower solids versions extend the film thickness range obtainable by a single coat process to less than 1 micron and above 200 microns. Casting of the solution can result in films of 1 to 2 mm or more in thickness. U.S. Pat. No. 4,882,245 also describes the application of these materials as a dry film photoresist when coated onto a carrier medium such as Mylar film.
A pattern-forming method based on X-ray lithography and which is called the “LIGA process,” is frequently used in the process of manufacture of MEMS devices (Kobunshi, High Polymers, 43, p 564 (1994). “LIGA” is a German term and is the abbreviation of Lithographie (lithography) Galvanoformung (electrodeposition) Abformung (molding). This process comprises the application of X-ray photoresists such as PMMA photoresists, an X-ray irradiation process, a solvent development process, a metal plating process and a resist pattern removal or “stripping” process. An alkali developable plating resist in which X-ray lithography is used has been proposed as disclosed in Japanese Patent Application Early Disclosure No. 9-90609 [1997]).
The “LIGA” process, which uses X-ray lithography, has the drawbacks that an expensive X-ray device is necessary, that X-ray irradiation takes a long time, and that development time is long. Because the X-ray LIGA process cannot satisfy the demands for saving resources, increasing workability, and increasing productivity which have become topics of discussion in recent years, there has been the desire for a LIGA process using less expensive and high throughput ultraviolet lithography systems. Many plating resists for use in UV lithography have been developed for use in printed circuit boards. Because they have comparatively small aspect ratios, there has been demand for new photoresists having characteristics suited for MEMS and micromachine fabrication.
Photoresists as disclosed in Japanese Patent Application Early Disclosure No. 5-45880 [1993], Japanese Patent Application Early Disclosure No. 6-204,150 [1993] and Japanese Patent Application Early Disclosure No. 11–315,384 [1999] are cited as photoresists for electroplating that can be processed by UV lithography.
While the SU-8 resin based compositions that have been disclosed are capable of very high resolution and aspect ratio, the cured resin has a tendency to be brittle, and often undergoes developer induced crazing/cracking, stress-induced cracking, has limited adhesion to certain substrates, and sometimes demonstrates delamination of the coating from the substrate. All these problems are exacerbated by the shrinkage induced stress that occurs when the material undergoes polymerization and is manifested in substrate bowing, where shrinkage of the coating induces bending (bowing) of the substrate. Further, SU-8 based photoresists must be developed with organic solvents or organic solvent mixtures and the imaged resists are so highly crosslinked after imaging that it is virtually impossible to remove the imaged photoresist with common strippers or other removal methods.
There have been many other prior art proposals for different photoimageable compositions including many that use epoxies. Examples of these can be found as referenced in U.S. Pat. No. 5,264,325. Here it is further taught that the photoresist material must be formulated such that it can be applied by coating methods, for example spin coating, which requires certain Theological properties. In addition, the composition must have the properties of providing sufficient transmission of the exposing radiation so as to photolyze the photoinitiator through the thickness of the film, and the photoresist must possess appropriate physical and chemical properties to withstand the application, such as electroplating or etch resistance, without significant degradation, or loss of adhesion.
Photoresist compositions that provide latent images that may be developed using aqueous solutions of organic and inorganic bases are of particular interest due to the interest in the electronics industry in using chemicals that more environmentally acceptable than organic solvents. U.S. Pat. Nos. 4,994,346, 5,079129, and 5,397,685 describe negative tone, epoxide containing, solder mask compositions that can be developed with aqueous base solutions. U.S. Pat. No. 5,120,633 describes photoresist compositions based on polymers bearing acid-labile protecting groups that are rendered soluble in aqueous base solution through the action of photochemically produced acids.
However, no specific type of epoxy resin has been found which will satisfy all of the various requirements; however many different combinations or mixtures of various epoxy resins have been disclosed. All of the noted patents describe various resins and photoinitiators for use in photocurable compositions, many of which are useful as photoimageable layers in permanent applications. However none of them teach or suggest the aqueous base developable, readily strippable compositions of the present invention nor are they suitable for the intended applications.
It is therefore desirable to provide a photoimageable formulation containing both epoxide groups and an aqueous alkaline soluble functionality in the same resin that may be cured with a photosensitive cationic initiator while retaining the good image resolution, high aspect ratio and straight sidewalls of SU-8 based photoresist formulations but at the same time allows the photoresist to be developed in a typical aqueous alkali based photoresist developer and allows the ready removal of the patterned photoresist after the post imaging processes have been carried out.